Parker O-Ring Handbook.pdf

3.1.2 Temperature
Operating temperature, or more properly, the range of system
temperature, may require some minor modifi cation of
the gland design. Gland dimensions given in the static and
dynamic seal design sections are calculated for the temperature
ranges listed for standard compounds. If the operation
is only to be at a high temperature, gland volume may need
to be increased to compensate for thermal expansion of the
O-ring. Conversely, for operation only at low temperature, a
better seal may result by reducing the gland depth, thereby
obtaining the proper squeeze on the contracted O-ring.
Table 2-4, which lists the approximate rate of linear thermal
expansion for typical elastomers and other materials, may
be utilized to calculate compensated gland dimensions. For
either high or low temperature seal designs, however, there
must normally be suffi cient squeeze to prevent leakage at
room temperature. Figure 3-1 illustrates another possible type
of design to improve low temperature sealing capability by
spring loading the O-ring.
Such special designs for high and low temperature environments
are seldom required. The minimum squeeze values
for the various O-ring cross-section diameters given in the
design charts of the static and dynamic seal design sections
are generally satisfactory.
Garter
Spring
Soft Metal Wedge
Figure 3-1: Spring-Loading for Low Temperature
Parker O-Ring Handbook
O-Ring
3.1.3 Pressure
Pressure has a bearing on O-ring seal design as it can affect
the choice of compound shore hardness. At very low
pressures, proper sealing may be more easily obtained with
lower durometer hardness (50-60 shore A). With higher
pressures, the combination of pressure and material shore
hardness determine the maximum clearance that may safely
be tolerated (see Figure 3-2). Cyclic fl uctuation of pressure
can cause local extrusion of the O-ring resulting in “nibbling”
(see Section X, Failure Modes), particularly if peak
system pressures are high enough to cause expansion of the
cylinder wall. One remedy may be to stiffen the cylinder to
limit the expansion so that the bore to piston clearance does
not exceed a safe value.
3.1.4 Extrusion
Extrusion of O-rings may also be prevented by the use of
anti-extrusion (back-up) devices. These are thin rings of much
harder material fi tted into the gland between the seal and the
clearance gaps, which essentially provide zero clearance.
They are available in hard elastomer compounds, leather,
PTFE, Nylon and other similar materials. Parker Parbaks ®
are elastomer back-up rings and are generally recommended
based on their proven functional superiority. The exact point
at which it becomes necessary to use anti-extrusion devices
will depend on the pressure, type of elastomer being used, its
Shore hardness, the size of the clearance gap, and the degree
of “breathing” of the metal parts which will be encountered.
Figure 3-2 may be used as a guide in determining whether
or not anti-extrusion rings should be used. When using the
data, include in the diametral clearance any “breathing,” or
expansion of the cylinder bore that may be anticipated due
to pressure. Although based on data obtained from O-rings,
the ninety durometer curve can also be used as a guide to
back-up ring performance. The Parbak Back-Up Rings Section
(VI), describes in greater detail Parker Parbak back-up
rings, and provides size and part number data. Also see “Patterns
of O-Ring Failure” in Section IX for more information
on extrusion.
Fluid Pressure (Bar)
Limits for Extrusion
690.0 10,000
552.0 8,000
414.0 6,000
276.0 4,000
207.0 3,000
138.0 2,000
69.0
Extrusion
1,000
55.2 800
41.4 600
No Extrusion
27.6 400
20.7 Hardness
Shore A
70 80 90
300
13.8 200
6.9
mm0
.3
.5
.8
100
1.0
In. 0 .010 .020 .030 .040
Total Diametral Clearance*
(Radial Clearance if Concentricity Between
Piston and Cylinder is Rigidly Maintained)
*Reduce the clearance shown by 60% when
using silicone or fluorosilicone elastomers.
Basis for Curves
1. 100,000 pressure cycles at the rate of 60 per minute from zero
to the indicated pressure.
2. Maximum temperature (i.e. test temperature) 71°C (160°F).
3. No back-up rings.
4. Total diametral clearance must include cylinder expansion due
to pressure.
5. Apply a reasonable safety factor in practical applications to allow
for excessively sharp edges and other imperfections and for
higher temperatures.
Figure 3-2: Limits for extrusion
Fluid Pressure (psi )
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
O-Ring Applications
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